The proposed studies address molecular mechanisms that may explain tissue-specific and gene- specific regulation by progestins. Aim 1 is to assess the functional differences between A- and B-receptors in breast and endometrial cells. Aim 2 is to investigate BUS (the B-upstream segment) as a third transcriptional activation domain unique to B-receptors. Finally, Aim 3 is to determine the mechanisms of TAF-3 action in the BUS segment.